This invention relates to an improvement of an internal combustion engine, particularly for a motor vehicle.
In conventional internal combustion engines for motor vehicles, since opening of a throttle valve is low and an amount of sucked air is small during idling and light loaded driving of the engine, the velocity of flow of the air-fuel mixture entering from an intake manifold into a cylinder is low during the suction stroke of the engine, thus the swirl produced in the cylinder is weak. As a result, during ignition, which normally takes place at the end of the compression stroke of the engine, the swirl of the mixture retained in the cylinder becomes weaker, so that firing and combustion of the mixture are not adequate. To ensure stable operation of the engine, it is necessary to supply a mixture having an air-fuel ratio which is lower than that of the mixture supplied during medium or high loaded driving of an engine, that is, the richer mixture is required. This results in an increase of fuel consumption as well as an increase in the amounts of carbon monoxide (CO) and hydrocarbon (HC) contained in exhaust gas due to imperfect combustion of the richer mixture.
Recently, it has been proposed to burn a mixture much leaner than the stoichiometric mixture in order to reduce the amount of CO and HC, and particularly the amount of nitrogen oxides (NO.sub.x) contained in the exhaust gas. Also, it has been proposed to draw a portion of exhaust gas from the exhaust system of the engine and to mix and burn it with the mixture in the cylinder to reduce the amount of NO.sub.x in the exhaust gas. However, in both cases, firing and combustion of the mixture became inefficient so that drivability and rate of fuel consumption became worse.